A retired doctor at the hospital has alleged that up to 40 veterans have died there because of these delays.

After veterans groups met with Shinseki urging him to "go public" about the allegations and to reassure veterans, he conducted a series of media interviews saying that he was angry about reported delays of care and any deaths that resulted and had sought an investigation by the agency's inspector general.

Shinseki dismissed calls by members of Congress and the American Legion, the nation's largest service organization, for him to step down.

The 71-year-old former Army chief of staff and decorated combat veteran — he received three Bronze Stars for valor and two Purple Hearts from serving in Vietnam — promised "swift and appropriate" punishment for any employees involved in medical delays and alleged coverups.

He also urged patience, allowing time for investigators to do their job.

USA Today reported this week that an internal VA investigation uncovered how employees at a VA outpatient clinic in Fort Collins, Colo., had been trained to falsify records so that wait times for patients appeared to fall within the agency's standard of 14 days.

The House Committee on Veterans Affairs issued the subpoena on a unanimous voice vote. It requested e-mails and written correspondence linked to the destruction or disappearance of any kind of waiting list at the Phoenix hospital.

The deadline for the VA to respond to this subpoena is May 19. After the deadline, the committee will review the materials provided by the VA and determine the next steps in its investigation.

The committee chairman, Rep. Jeff Miller, R-Fla., said the the panel had made calls and written a letter seeking information about the documents but did not receive a satisfactory response.

"The last few weeks have been a model of VA stonewalling, which precipitated the need for a subpoena," Miller said.